OK I see, it looks like that's up high, so you can't use it as a drain. Too bad. Maybe you can splice in a drain valve at the lowest hose, to make it easier for the future. PEX is easy to work with and PEX fittings are common household plumbing items. Then you could use air to blow out the rest of the system.

BTW using air, it really doesn't matter if a few small plugs of water collect at the lowest points in plastic tubing. If you blow it out completely any little pockets that collect won't expand enough to the side to affect the flexible plastic, and a little pool of ice in the bottom of the water tank or expansion chamber will have room to expand without harm. It's important to leave valves and thru-hulls in the open position so they don't trap water, and any hard fittings such as hose barbs at the bottom of water tanks are vulnerable (so you must be sure those are clear).